Electric resistance, gas-tight furnace



Jan. 16, 1962 G. FEGAN & 2

ELECTRIC RESISTANCE, GAS-TIGHT FURNACE Filed July 30, 1957 5 Sheets-Sheet 1 INVENTOR.

&232

Jan. 16,` 1962 G. J. FEGANV & 73 2 ELECTRIC RESISTANCE, GAS-TIGHT FURNACE Filed July 30, 1957 5 Sheets-Sheet 2 INVENTOR.

Jan. 16, 1962 G. J. FEGAN 3,017,262

ELECTRIC RESISTANCE, GAS-TIGHT FURNACE Filed July 30, 1957 5 Sheets-Sheet 3 INVENTOR.

Jan. 16, 1962 G. J. FEGAN & 3

ELECTRIC RESISTANCE, GAS-TIGHT FURNACE Filed July 30, 1957 5 Sheets-Sheet 4 /400 Low CARBO/V TEEL [200 mamane 17\ /000 ELFMENT un' cum/E 17 .2 gg/7 7% K 7/4* Jan. 16, 1962 G. J. FEGAN ELECTRIC RESISTANCE, GAS-TIGHT FURNACE 5 Sheets-Sheet 5 Filed July 30, 1957 TFMPERATUEE "c f/VVEIVTOR.

United States Patent O 3,017,262 ELECTRIC RESISTAN CE, GAS-TIGHT FURNACE George J. Fegan, Spokane, Wash., assignor to Chromium Mining & Smelting Corporation, Limited, Sault Ste. Marie, Ontario, Canada, a corporation of Ontario Filed July 30, 1957, Ser. No. 675,053 6 Claims. (Cl. 75-28) This invention relates to an electric furnace wherein heat is generated by exposed resistance conductors or filaments connected across a power line. More particularly, it relates to a combinaton of a gas tight furnace casing and heating elements which are subject to corrosion or oxidation but which are capable of functioning at high temperatures when protected by a suitable atmosphere such as a high vacuum or an inert gas. Still more specifically, it relates to such a furnace which is intended to be operated at temperatures in excess of 1150 C. on relatively long power cycles.

Before setting forth the objects of this invention, applicant will describe a conventional vacuum furnace so that references to such a furnace may be understood. Since the drawings are used in this description, they will first be described. The figures are:

FIGURE 1 is a section through a vacuum furnace;

FIGURE 2 is a View taken on the line 2-2 of FIG- URE l;

FIGURE 3 is a schematic wiring diagram of the automatic controls of such a furnace;

FIGURE 4 is a schematic illustration of the wiring connections of the heating straps as they appear on the inside of the furnace wall;

FlGURE 5 is a view transversely through the walls of the furnace showing the mounting of a hanger;

FIGURE 6 is a chart comparing the loading characteristics of Nchrome V and low carbon steel;

FIGURE 7 is a chart comparing the aging characteristics of Nchrome V and low carbon steel; and

FIGURE 8 is a chart comparing the rise in electrical resistance of steel having a carbon content of &OG-038%, graph 1; high alloy steel containing .OS-.37% magnesium, 19.l% chromium, &14% nickel, and 0.6% tungsten, graph 2; Nchrome V, graph 3; and Kanthal A-l, graph 4 as their temperature is raised.

Referring to FIGURE 1, the numeral 10 identifies a gas-tight casing having a bottom 12 and a cylindrical side wall 14. This casing is lined with an insulating material 16, as for example brick. A lid 18 is mountable upon the casing 10 in gas-tight relationship at 20. A pipe 22, which may be connected externally to a gas eX- haust system, connects the interior of the furnace with the exterior. Six banks of sinusoidal conductors or filaments 38, 40, 42, 44-, 46 and 48 having a one by oneeighth inch cross section, are mounted around the inside wall of the chamber 28 and are connected by leads such as 30 and 32 to a source of electric power. Trays, such as 34, for holding material to be heated, for example ferrochromium, are stacked within the furnace. These trays are lifted by crane out through the opening 36 at the top of the heating space 28. There may be a secondary lid 38.

The controls and connections to the heating elements are generally indicated in schematic FIGURES 3 and 4. A power line is indicated at 50 and this is tapped by a hot line conductor 52 and a ground conductor 54. C is the main power contactor and when it is closed, current is led through the conductor 56 to the two banks of heating elements 42 and 44 and thence by the conductor 58 to the ground side of the line, conductor 54. The conductor 56 is also connected to the banks of heating elements 46 and 48 which are connected by conductor 60 through contactor A to ground 54. Similarly, conductor 56 is connected through contactor B to heating elements 38 and 40 which are connected by conductor 58 to ground conductor 54.

A time relay is indicated by the panel 62. This panel derives power from a 120-volt D.C. source comprising a hot line 64 and a ground 66. Upon the closing of a switch 68 in the D.C. power circuit, relay 70 closes thereby closing the switch C which is identical to the contactor 'C in the lower part of the figure. After thirty minutes a time delay relay 72 closes and this closes the contactor A which is identical to the contactor A shown in the lower left hand side of the figure. Thirty minutes later, time delay relay 74 closes and this closes the contactor B which is identical to the contactor B in the central part of the figure. The time delay relay is equipped with reset relay switches none of which interests us here.

Disposed within the furnace is a thermal element 76 which controls a switch in a control 78 which is set to open and close the main power contactor C at the top and the bottom of a selected range of temperatures within the furnace. Once the time delay relay has gone through its complete cycle, the heating elements are under the control of the switch 78 until the end of the furnace cycle. When the furnace reaches the maximum temperature, switch 70 opens. The connections through the time relay 62 are such that only the switch C is opened while B and A remain closed. When the temperature in the furnace drops to the point such that the'switch 80 is closed, C is again closed and all of the heating elements 38-48 are simultaneously energized. This control equipment is all standard. It includes parts such as reset relay switches which do not concern us here. This furnace and control equipment is standard. It has not been described in detail as it does not concern this invention except in the general combination. Before this invention, the heating elements used in the furnaces of applicant were Nchrome, which is 60% Ni, 15% Cr, and 25% Fe, and for applicant's purposes, more generally they were Nchrome V which is 79 /2% Ni, 20% Cr, and /2% Fe. These are five to ten ton furnaces and a complete set of heating elements cost about $1000. Because the heating elements were highly non-corrosive and resistant to oxidation, no precautions were taken in evacuating the furnace before energizing the heating elements.

The general object of this invention is to provide a furnace which may be operated at temperatures up to 1300 C. for long periods of time. The element in the furnace that gives way was the heating element, or the terminal connections to those elements. The manufacturers of Nchrome and Nchrome V state that the heating elements are not intended for operation at temperatures substantially above 1100 C. On the other hand, the reduction of high carbon ferrochromium to low carbon ferrochromium in a vacuum furnace progresses best at temperatures of l200-1300 C.

In trying to improvethe heating elements, analysis was first made of the functioning of the Nchrome V which was the most expensive and the most satisfactory of the heating elements theretofore used by the applicant. These heating elements failed in two ways.

Firstly, there is a gradual increase in resistance which reduces the furnace power below a satisfactory level in about one year`s time. This is referred to as its aging characteristic or its resistance-property. It has a very important hearing on the design of a furnace, particularly the control elements and the power input. Referring to FIGURE 7, the life of a Nchrome V element is charted along the X axis, while the amperage which it can pass is charted on the Y aXis. The temperature rise in a furnace is controlled by the total power input. It follows that the heating elements, the controls and the maximum power input must be selected so as to provide the req- 5 uisite furnace load during the period of highest resistance of the heating elements. FIGURE 7 shows that this occurs late in the life of Nichrome V. It follows that if during the last 20% of the life of a Nichrome V element, it will pass 1100 amperes or less, and if this is sufficient to maintain the temperature desired, then the same voltage would be capable of forcing through 1600 amperes when the elements Were new and their electrcal resistance lower. FIGURE 7 shows that the rise in resistance of Nichrome V due to aging s steady. Nichrome never levels off, so that with the voltage constant, power input continuously declines. Phrased differently, sixty percent more voltage is required at the end of the period than at the beginning, and the equipment must be designed to gradually increase the voltage over the life of the elements.

Secondly, the Nichrome V heating elements develop local brittle spots which cause complete failure in as little as 10% of ts normal life. Where these occur in a comparatively new element, the element is repaired. The rise in resistance is probably due to grain growth which is also responsible for weakening the element. The elements are on power approximately 40% of the total time, after Operating temperature has been reached.

Thirdly, the resistance-temperature property of Nichrorne V results in a disadvantage which is greater than that obtained by its straight-line characteristic which is considered one of its great assets. Referring to FIG- URE 6, it Will be seen that its ability to carry load does not vary appreciably up or down due to its own temperature. See also curve 3 of FIGURE 8. This was pointed to as an advantage in designing the special electrical control equipment which limited in-rush of current When the heating elements are initially energized. Applicant came to realize, however, that this very property caused the furnace to cycle frequently after the furnace had reached Operating temperature. Let us suppose that the automatic temperature controls are set to open the electric circuits at 1210 C. and to close them at 1l90 C. In the furnaces that applicanfs assignee is using for converting high carbon ferrochromium to low carbon ferrochromium, the total amount of time that the circuits are closed is approxmately 40%, that is twenty-four minutes out of each hour. While it does not matter so far as the material being treated is concerned whether the heating elements are on for two minutes and oti? for three, or on for four minutes and off for six, the first of these two Operations is twice as wearing on the controls as is the second. The constant resistance of nickel chrome alloys at Operating temperatures results in rapid heating, thereby shortening the heating cycle, which results in more frequent functioning of the controls.

The first specific object of this nvention is to find a material for the heating element straps which will withstand higher temperatures than Nichrome. This material must be sufiiciently ductile or malleable so that it may be bent over the hangers as illustrated in FIGURE 4. It must have a high melting point that is well above 14S0 C. Importantly, because the furnace is gas-tight, it need not be resistant to corrosion or oxidation and it must have a high vapor pressure to withstand vaporizaton at the furnace temperature and pressure.

A second specific object of this invention is to find a material which will have a superior resistance-time property, ie., a more constant resistance to electrical conductivity during its entire life so that the efficiency and the range of power load of the heating element will be more nearly uniform as between its early input and its late-in-life input. This saves on controls and total 'length of heating element.

Another specific object of this invention is to find a material whose resistance-temperature property is such that the special electrical controls to limit inrush o-f power upon initial heating are aided, and after the furnace has reached control temperature, the cycling of the controls will be less frequent.

In searching for an improved material the applicart discarded all metals and alloys having a melting point below 1100 C. Those considered by applicant are set forth in Table II below. These were rejected. He then considered the metals set out in Table I below. He rejected all of the alloys commencing with Nichrorne V in Table I because of their high cost. Also, being alloys they have comparatively low melting points. With respect to the pure metals, manganese was rejected because of its low melting point and still lower vaporization point. Carbon was discarded because it tends to spall and also to contaminate the product in the furnace. It is presently in use for heating elements in vacuum furnaces. Tungsten, chromiurn, nickel and tantalum were rejected as too costly. This left silicon, iron, titanium and molybdenum and since iron was so much the cheapest, experiments progressed with iron Table I and II follow:

TABLE I Resista'ce materials which mm; work above 1150 C'. in vacuum Alloying Elements Vapor pressure Melting Remarks 76 .,Hg, Point, C.

Ni Or Fe Mn Si C Temp. C.

- 100 3,257 3, 700 Carbon.

3` 547 3, 410 Tun sten.

3. 900 2,850 Tantalum.

2, 727 2, 625 Molybdenum.

1,420 1, 890 chromiurn,

1,990 1.820 Titanum.

1,564 1, 537 Iron.

100 1, 586 1,455 Nic-kel.

100 1, 572 1. 430 Silcon.

1,115 1,245 Manganese.

80 20 0.5 25 1, 400 Nichrome V. 10 2 5 'I 1, 350 Nlchrome.

35 20 romax.

13 (n Al and Fe '2 1, 395 Inconel.

36 1. 425 Nilvar.

46 1, 425 #46 Alloy.

8 1, 399 #1304 Stainless ee 37. 5 55 7.5% Al 'I 1, 510 Kanthal.

*Small amount.

TABLE II Resstance materials which are 'not suitable for aver 1150 C'. in vacuum Alloying Elements Max.

. Operat- Melting Remarks ing Temp Point,

18 27 Zinc 260 130 1, 020 Mangonn.

23 540 1, 100 Midohm. Bal. Additions... 1,100 Permanickel.

6 315 1,100 Lohm.

Cu and Zinc. 930 High Brass. Cu and Zinc 1, 100 Low Brass. 213 315 1, 100 Alloy 30. Cu and Zinc 1, 040 Bronze.

12 88 430 1, 100 90 Alloy. 99 and Al 210 660 Aluminum. 99. 9 315 1, 083 Copper. 70 30 Zn 370 l, 000 Brass.

Experments Were made with three types of iron. The t postponed loading an evacuatng system which Was first was ordinary mild steel being SAE 1015 Steel strip, one inch wide and one-eighth inch thick. These dimensions were common to all of the strips. T=his steel contains .15% carbon when new. The second type Was made of wrought iron whose carbon content was 006% to 008%. A third strip was made of Wrought iron. Initial tests used nine production runs as the measure of sufliciency. If the unit did not Survive this period of time, it was unsatisfactory.

Analyses were made on the iron and steel heating elements after production runs with ferrochrome under vacuum. It was found that the carbon content of the iron and steel had decreased to .01% to .02% by Weight and there was a definite chromium pickup in the heating elements, the chromium being present mainly on the surface and decreasng toward the interier. The surface scale from the heating elements Was about 4% by weight chromium while the clear surface metal scraped clean and then a file sample taken was 13% chromium after nine runs.

It is believed that the long life of the iron and steel heating elements at the high temperatures used in production is due to the low carbon content of the metal and the fact that it has a surface layer containing chromium. This occurs during the production runs so that after the first run, and even during the first run, the heating elements consist of iron containing much less carbon than present in the element before use, and have a surface layer containing a substantial amount of chromium.

With ferro manganese, the carbon content also decreases and a surface layer of iron containing manganese is formed. Long life at high temperatures is also attained.

The preferred heating elements of this invention essentially consist of iron or steel having a carbon content of .15% or lower. With such low carbon steel, or With Wrought iron having lower carbon content, the carbon content of the iron quickly diminishes to below .03% carbon and the surface of the element becomes protected by reason of substantial chromium pickup of for example to 5% chromium and sometimes higher. Iron or steel having a higher initial carbon content such as .5% can also be used but the time for depletion of the carbon content to the preferred amount of not more than .03% by weight is slightly greater.

All of the testing was done under one of two conditions-either with a substantial vacuum or a protective gas in the furnace. Considering the vacuum treatment first, when the Nichrome V elements were used, the practice Was to energize the electric heating elements until the temperature had reached several hundred degrees centigrade before opening the valve into the vacuum system. Since the Nichrome elements were resistant to corrosion and oxidation, this was justified simply because handling a large number of furnaces. With the iron heating elements, it is still not necessary to commence evacuation before energzing the heating elements but it is desirable to commence evacuation when the tempera` ture of the furnace exceeds 350 C. Experiment shows that the degree of evacuation which will adequately protect the element is five inches of mercury absolute up to a temperature of 1300" C. In actual practice, the degree of evacuation is higher, about one inch absolute When the temperature passes the 1000 C. mark.

EXAMPLE I ordinary mild steel strap having a one-inch by oneeighth inch cross section and provided on a reel was given the sinusoidal configuration shown in FIGURE 4 outside the furnace. One end was then inserted in a terminal post 90, referring to FIGURE 4, and the strap was belayed around the inside of the circle over the hangers 92, 94, etc., to form bank 38, and the end was inserted in the electrode 96. The preferred method is to have the hangers 92, 94 already in place With the belaying done With the sinusoidal or pre-shaped strap. It is possible to drive the hangers 92, 94 into the insulating material 16 as required, see FIGURE 5.

The steel used Was SAE 1015. This steel contains .15% carbon. The carbon content after the first run described below was .02% carbon, and the chromium content of the surface, after removing scale, was 1 5% chromium by weight, the remainder being iron. Ductile iron or steel having an internal carbon content of not more than approximately .4% by weight has been found satisfactory.

The operation of the furnace difers from that where nickel-chromium alloys are used for the heating elements in that the valve opening the furnace into the vacuum system is open when the temperature passes 350 C. in order to protect the iron elements. This procedure is no more costly. The temperature of the furnace with load reaches the selected maximum within a few hours, and While under the old practice, no load was imposed upon the evacuation system for a longer period of time, this load imposition is not of great consequence. It is important that the steel of the heating element be not oxidized. The etfect of the heating elements on the furnace loading is shown in FIGURE 6. The initial load Was something short of 900 amps., utilizing just the first two banks, 42 and 44 of FIGURES l and 3, as the iron became hotter, the resistance increased, and as FIGURE 6 shows, at the end of half an hour, the furnace load had dropped to 600 amps. Thereupon, the time control cut in heating banks 46 and 48. There was a momentary surge up to 2200 amps., which almost immediately dropped back down to 1400 amps. During the next half hour, the resistance of the iron continued to rise and the furnace load declined to about l300 C.

After one hour of operation, the last two banks of heating elements, namely 38 and 40, are cut in, which again causes a surge of furnace load up to about 2200 amps. and then this declines hour by hour until after about twenty hours, the resistance of the iron becomes substantially constant.

It should be observed that the resistance characteristc of the iron aid the special control which limits in-rush. As the iron electrodes temperature rises, by their own resistance temperature characteristics, they cut down the furnace load. Referring to FIGURE 6, note the decline after the initial peaks 100 and 102. lt will be appreciated that this curve of FIGURE 6 for the low carbon steel, or after a short period of time substantially pure iron, reduces cycling of the control apparatus Which governs in response to the terminal element in the furnace.

The second great advantage lies in the aging characteristics of low carbon steel. Referring to FIGURE 7, the decline in conductivity is comparatively slow and the range is not great, whereas Nichrome V over its life drops approximately 40%, low carbon steel drops only about 15%. It follows that whereas means must be provided for increasing the voltage by 40% during the life of a furnace equipped with Nichrome V heating elements, the equipment required for a furnace using low carbon steel heating elements need only raise the voltage by about 15%.

As for the condition of the Steel during and at the end of its life, it becomes substantially pure iron having a carbon content below .03% on the first heating with a chromium or manganese coating surface layer of 1% to 5% chromium or manganese. The carbon in the steel combines with oxygen or otherwise escapes. The chromium or manganese pickup comes from the ferrochrome or ferromanganese, probably by decomposition of vapors of these metals present during the vacuum operation. The physical character of the heating element changes from one of ductility or malleability so essential for easily forming it into the sinusoidal shape to one of considerable brittleness. However, the change is not suflicient to cause parting of the material, that is, breaks either at the hangers or in the flights between the hangers. It is noticeable that the iron because of its much higher melting point does not develop the local brittle spots which sometimes cause the chromium-nickel alloy heating elements to develop a break before Operating of its normal life. The grain size of the steel increases at a much higher speed early in the life of the element than in its later life. lt is believed that resistance Varies With the grain size and that the flattening off of the curve in FIG- URE 7 of the low carbon steel indicates that at the end of about one-quarter of its life, the speed of increase in the grain size is reduced and increases only gradually but constantly.

Referring to FIGURE 7, it will be noted that both the low carbon steel and the Nichrome V are plotted against their life in terms of whole life of each. The conclusion is not to be drawn that their lives are equal, but the surprising point is that the low carbon heating element has a suficiently long life for practical use, and not substantially less than Nichrome V. It is profitable to use the low carbon steel heating elements if they have a life of at least onc-fourth that of the nickel chrome alloy. The life of the low carbon steel heating elements is at least twice this minimum satisfactory life.

EXAMPLE II A portion of one furnace was equipped with a wrought iron heating element having the same dimensione as the steel heating element. Its carbon content was .07% by weight. Similar tests were made, but no appreciable difference in results were obtained. A heating element made of wrought iron will ordinarily not be used because it is much more expensive, having to be made in the cross section required and it is not nearly so easy to shape and mount in the furnace.

EXAMPLE III This Was a test in which the strap was ordinary malleable iron i.e., capable of being hammered or shaped by 'hammering or by pressure. This iron Was much higher in carbon content being in the range of &ZO-030%. The results were the same as with the wrought iron, and the malleable iron would also probably not be used for practical reasons.

Efiect of carbon in the iron The applicant has experimented with steels having a carbon content of as high as 032%. Insofar as the carbon renders the steel bendable so that it will hold its shape while easily belayed over the hangers, the carbon is useful. otherwise, it might as Well be absent. An analysis of the carbon content of the steel used in Ex ample I was made after several production runs and it was found to be .01 to .02%. Theoretically, the less carbon in the metal, the better because the heating element will be less porous after the carbon has been removed during the furnace operation.

Efiect of chromium or manganese The chromium and/or manganese present in the surface layer of the heating element strengthens the heating element and increases its life at high temperatures, by rendering it less subject to corrosion and oxidation. Single runs have been made with satisfactory results without chromium or manganese being present but for several runs it is preferred to have the chromium or manganese present in the surface layer (l mm. or less) of at least .5%. The chromium or manganese content of the surface layer increases with the number of production runs and has been found to be about 12% for seven runs and 1.8% for nine runs.

Practice on newly bricked furnace Whenever the elements are installed in a newly bricked furnace, there is much water present in the brick and a drying run equal to a standard production cycle is required to drive out all of this moisture. During such a run, a contaner of coke (about pounds) is placed in the furnace to act as a getter for the oxygen in the water which is decomposed by the heat and 'low absolute pressure. By following such a practice, the element is unatfected by these runs even with the moisture present.

The atmosphere that bre /cs the elements It is essential that the elements be not operated at a high temperature in the presence of oxygen or other damaging elements. In the reduction of high carbon ferrochromium the protection is provided by a Very high vacuum. On a 100-hour cycle, there will be long periods of time when the vacuum will be in a range below 100 microns. However, experiment has shown that for at least the short periods of time no serious damage is done to the elements were the pressure is as high as five inches of mercury absolute and the temperature in excess of 1100 C. The vacuum as a means of producing the elements is of course used where it performs some other function, such as creating a low vapor pressure in a product in which one is seeking to remove carbon by uniting it with oxygen.

In those applications where a reaction is to take place in an electric furnace at very high temperature, applicant's resistance elements can be further protected by using any inert gas such as argon, helium, krypton, neon, Xenon or Radon, provided the gas is substantially unmixed with oxygen or some other damaging element. As in the case of the. vacuum furnace, the casing must be tight. If the reactin materials are themselves generating an element damaging to the iron, it must be removed.

Usually, a reducing atmosphere consisting of hydrogen or nitrogen or one of the carbon oxides will suflice. There are applications where hydrocarbons in gaseous form may be used as a protective atmosphere.

Operating temperatures In each of the above examples, the furnace was operated at temperatures up to 1300 C. This is some 200 C. above the temperature recommended for Operating furnaces having nickel-chromium alloy heating elements. The temperature must not go much higher because even short of reaching the melting point of the iron, it becomes soft and loses suflicient tensile strength to maintain itself on the hangers.

Advantages The advantages of applicant's steel heating elements are several. Firstly, the furnace can be operated at from 100-200 C. higher temperature without developing prematurely Weak and burned-out spots in the heating elements. Secondly, the nature of Inild steel is such that it is easy to bend and consequently the configuration of the heating element may be established outside of the furnace and then the entire heating element assembled on the hangers already in the furnace. This is much superior to the comparatively brittle nickel-chromium alloy heating elements. Thirdly, the fact that iron has the characteristic of resisting electric conductivity as its temperature rises tends to aid the automatic controls which prevent initial load in-rush, and to reduce Cycling of the automatic controls after Operating temperature has been attained. The fourth advantage arises from the fact that the range in the resistance rise to electric conductivity of iron over the aging of the heating element, see FIGURE 7, is comparatively narrow so that in designing the furnace in the first instance, the power source conductors, etc. necessary to handle an increased electrical load near the end of the life of the heating elements is only slightly greater than what is required at the beginning of their life. Finally, the cost of the iron heating elements is a great deal less than nickel-chromium heating elements, and while this advantage has not fully been realized, further improvements offer a great possbility to this.

Having thus described his invention, what applicant claims is:

1. In an electrc resistance furnace having a gas-tight chamber with means for maintaining selected sub-atmospheric pressures of any gas therein and wherein temperatures in excess of 1100 C. are maintained by bare resistance conductors, the improvement comprising conductors composed of a material selected from the group of wrought iron and carbon steel which have a carbon content of not more than .03% by weight and have a surface layer of iron containing at least .5% of a metal of the group consisting of chromium and manganese.

2. A heating element consisting essentially of iron containing not more than .03% by weight of carbon and having a surface layer of iron containing .5% to about 5% by weight of a metal of the group consisting of chromium and manganese.

3. In a process of decreasing the carbon content of an alloy of the group consisting of ferrochromium and ferromanganese by heating the alloy under vacuum in an electric furnace at temperatures exceeding 1100 C. having exposed resistance conductors, the improvement which comprises heating the alloy under vacuum with resistance conductors selected from the group consisting of wrought iron and carbon steel having a carbon content of not more than 0.4% by weight.

4. In a process of decreasng the carbon content of an alloy of the group consisting of ferrochromium and ferromanganese by heating the alloy under vacuum in an electric furnace at temperatures exceeding 1100 C. having exposed resistance conductors, the improvement which comprises heating the alloy under vacuum with resistance conductors selected from the group consisting of wrought iron and carbon steel having a carbon content of not more than 03% by weight, and having a surface layer of iron containing at least 05% of a metal of the group consisting of chromium and manganese.

5. The process of claim 4 wherein the vacuum s below five inches of mercury absolute.

6. The method of building an electric resistance furnace having internally exposed linear conductors which comprises the steps of belaying over hangers linear conductors composed of a ductile ferrous metal selected from the group consisting of wrought iron and carbon steel, of connecting said conductors to a source of power, of charging the furnace With a material selected from the group` consisting of ferrochromium and ferromanganese, of reducng the pressure in the chamber to a point well below atmospheric pressure, and then of heating said conductors under vacuum conditions so as to reduce the carbon content of said conductors to less than .03%.

References Cited in the file of this patent UNITED STATES PATENTS 1,184,706 Keller May 23, 1916 1,233,183 Carter July 10, 1917 1,603,165 Swoboda et al Oct. 12, 1926 1,678,875 Rohn ..a July 31, 1928 1,980,825 Rankn Nov. 13, 1934 2,473,019 Erasmus June 14, 1949 2,473,021 Spendelow et al June 14, 1949 OTHER REFERENCES Trinks: Industrial Furnaces, vol. II, 2nd ed., 1942, pages 132, 134, and 139; published by John Wiley and Sons, Inc., New York, N.Y. 

1. IN AN ELECTRIC RESISTANCE HAVING A GAS-TIGHT CHAMBER WITH MEANS FOR MAINTAINING SELECTED SUB-ATOMSPHERIC PRESSURES OF ANY GAS THEREIN AND WHEREIN TEMPERATURES IN EXCESS OF 1100* C. ARE MAINTAINED BY BARE RESISTANCE CONDUCTORS, THE IMPROVEMENT COMPRISING CONDUCTORS COMPOSED OF A MATERIAL SELECTED FROM THE GROUP OF 